Cathode ray tube system



Feb. 15, 1944. J. DE GIER ,3

CATHODE RAY TUBE SYSTEM Filed Dec. 2, 1941 INVENTOR Jamie Gwr ATTORNEYPatented Feb. 15, 1944- chm-noon any 'runn SYSTEM Johannes de Gier,Eindhoven, Netherlands; vested in the Alien Property CustodianApplication December 2, 1941, Serial No. 421,310 In the NetherlandsMarch 9, 1940 8 Claims. (Cl. 315-22) My invention relates to electrondischarge tubes and systems of the cathode ray type and particularly toan improved electrode structure for use in such tubes and systems.

In cathode ray tubes of the types utilized for recreation of televisionimages and for oscillagraph purposes it is desirable to provide astructure capable of developing an electron beam of high electrondensity while atthe same time limiting the cross sectional diameter atthe target to a value as small as possible. High electron beam currentmay be obtained while retaining a small beam cross section at the targetor fluorescent screen by focusing the electron streams from the cathodewith the'aid'of a pre-concentration system and by projecting a sectionof the beam thus formed on to the screen by means of an electrostaticlens system. In this case it is desirable to utilize an apertureddiaphragm serving to limit the cross-sectional 'area of the beam of raysin order to avoid stray electrons impinging on the target or screen inthe environment of the picture proper and producing additional light orhalation over the target. Such a diaphragm has the disadvantage that theelectrons which are intercepted produce secondary electron emission andif no particular provisions are made the secondary electrons thusliberated impinge not only on the target but also on other electrodessuch as the deflecting plates and cause a variation of the potential ofthese plates if they are connected to a scanning circuit through acomparatively high resistance. This has the troublesome eifect that thepicture point on the screen varies its position with the current and thefigure described by this point is deformed. The influence of thesecondary emission is reduced if the apertured diaphragm is placednearer to the cathode. Thus if it is secured to the first electrode oranode of the electrostatic projection system, then nearly all theelectrons liberated by secondary emission are collected by theelectrodes of, the projection system, especially if the diaphragm islocated in an electrostatic field free space. In this case, however,another dliliculty is encountered in that the electrons which impinge onthe diaphragm have the effect of reducing the potential of theassociated anode which is usually connected across a comparatively highresistance to a source of potential. This results in a fluctuation ofthe I anode potential and of the focal distance of the projection systemand this causes defocusing of the image on the target screen.Consequently, with variation of the beam current, the voltage ratiobetween the electrodes varies. and must be adjusted subsequently, inorder to keep the image sharp and in focus. g

It is an object of my invention to provide a cathode ray tube and systemwherein the electron beam may be maintained in a focused conditionnotwithstanding wide variations in beam current. It is another object toprovide a cathode raytube-system utilizing an electron gun having trolthe variation of the focal distance of the electron beam in such amanner that the disadvantages referred to above are eliminated. Theseand other objects, features and advantages of my invention will becomeapparent when considered in view of the following description and theaccompanying drawing, in which:

Figure l is a longitudinal view of a cathode ray tube system havingelectrode structure made and operated in accordance with my invention,and

Figure 2 is a cross sectional view of one of the electrodes shown inFigure 1 taken along the line 2-2.

I have found that it is almost exclusively those secondary electronswhich emanate from the margin of a diaphragm in the and electrode whichbring about troublesome phenomena, such as shift of the picture pointand diffused screen light.

Therefore, in accordance with my invention, I provide a structurewherein the electron beam is limited by an apertured diaphragm which islocated at such a point that it does not emit harmful secondary andreflected electrons but transmits only those primary electrons whichcannot generate any appreciable harmful secondary radiation from furtherelectrodes of the structure.

Further, in accordance with my invention I provide a diaphragm whichcollects only the stray electrons which directly adjoin the electronbeam which is to be utilized to generate light on a target such as afluorescent screen. .Thus the stray electrons can no longer impinge on afurther electrode at a position which might cause further secondaryelectron emission and. therefore no serious trouble is encountered fromsecondary electrons emitted from this electrode. The potential of theapertured diaphragm, and associated anode limiting the rays andconsequently the focal distance of the projection system are variable toa smaller degree since a great portion of the rays which are not used isnot intercepted by the diaphragm and consequently does not contribute tothe voltage variation of the diaphragm.

The invention will be more clearly understood with reference to Figure 1wherein the highly evacuated envelope E is of elongated shape providedwith a neck section enclosing an electron gun made in accordance with myinvention and a frusto-conicai section enclosing a target elec- Due tothe electric field between the cathode I and the first anode 3, theelectrons are accelerated through the control electrode 2 which has anegative voltage relative to the cathode causing the electron paths toconverge toward the electron gun axis to form an electron beam 6. In thetube represented in Figure 1 the voltages applied to the electrodes havesuch values that the paths of the electrons emitted from any point ofthe emitting surface of the cathode intersect or cross over each otherin the vicinity of the first anode apertured disc 4. Consequently, theintersection of the electron paths to form the beam 6 is not at amathematical point but over a small surface atwhich the beam has aminimum cross section. After the electrons of the beam 6 pass thecross-over point they follow divergent paths and are subsequentlydirected by the electrostatic fields between the first and second anodeson to the fluorescent screen S. While I have shown a structure in whichthe electrons converge from the cathode at a crossover point, theelectrons may alternatively diverge from the cathode. The first anode 3and second anode are maintained at positive potentials with respect'tothe cathode I, preferably by a single potential source 8. Since thefirst anode is operated at a lower potential with respect to the cathodethan the potential of the second anode, I provide a bleeder orpotentiometer 9 of high resistance shunted across the source 8 to derivethe potential applied to the first anode 4 such as by the variabl leadII) from the source 8.

In accordance with the invention, and in order to prevent excessivevariation of the ratio of potentials existing on the first and secondanodes, I provide a multi-apertured diaphragm II, which is so shaped asto intercept only part of the rays which are not used. As can be seenmore clearly from Figure 2, the diaphragm II has a central aperture I2which is aligned with the axis of the electron gun and a plurality ofouter apertures I3 011' the electron gun axis. The central portion I4 ofthe diaphragm is preferably of annular shape and intercepts the rays ina region which directly adjoins the beam, transmitting through theapertures I3 the rays running in a marginal region I5 which is stillfurther away from the axis. vThe latter rays consequently reach theelectrode 5 but do not produce any harmful secondary emission since theyimpinge on the electrode at points from which the secondary electronscannot penetrate through the aperture I6 in the diaphragm I. Thesecondary electrons which are produced at the central portion ll of thediaphragm II do not bring about disadvan tageous phenomena since thediaphragm is positioned in an almost field-free space so that thesecondary electrons are not accelerated in th direction of the apertureI5. I

While wide fluctuations of the potential of the first anode 3 withvarying beam intensities are eliminated, due to the particular shape ofthe diaphragm II, some residual variation in this potential due to thecollection of electrons by the central annular portion is desirable. Theintensity of the beam is controlled by setting up a voltage betweenpoints I! and I8 in the connection between the control electrode 2 andthe cathode I, which voltage renders the electrode 2.

negative with respect to the cathode. The greater this potentialdifference, the weaker is the beam current but also the more arecompressed the rays emanating from the cathode. Consequently, thecross-over point of the beam electrons adjacent the apertured disc 4will be located nearer to the cathode at comparatively high modulationvoltages. However, the residual variation of the first anode potentialcompensates for any defocusing of the electron beam and the focaldistance of the lens system is rendered substantially constant withvariation of electron beam intensity. It is very desirable that the areaof the central portion I4 and of the apertures I2 and I3 of thediaphragm II should be chosen to collect a portion of the electron fiowthereby obtaining some slight variation of first anode potential withvarying beam current. In conventional circuits this variation of thefirst anode potential and of focal distance is far too great with adiaphragm which intercepts all the marginal rays so that thedisplacement of the cross-over point of the electrons is overcompensatedin this case. The ratio of the surface of the annular Portion I4 toaperture I3 area may be determined by trial but in any case apertures toallow marginal electrons to pass to the second anode must be provided toobtain the benefits of my invention. The diaphragm I I may also beconstituted by a plurality of concentric rings or it may have a helicalportion surrounding the annular central portion I4. The essential thingis that a central portion of the beam is transmitted and a portion ofannular section about itis intercepted and outside the annular portion aquantity of rays is transmitted.

In the application of cathode ray tubes to oscillography it is oftenpossible for the electrode voltage to be adjusted during use and thus tokeep the beam in focus. In many cases, however, especially in recordingof transient phenomena the short duration. of the surges to be measurednecessitates an automatic control which makes the use of my inventionparticularly desirable.

While I have described my invention with particular reference to acathode ray tube of the fluorescent screen type it will be understoodthat it is equally applicable to tubes of the television transmitting orlight valve types wherein different targets are utilized and I,therefore, do not wish to limit my invention to the particular structuredisclosed or the uses to which it may be applicable except as set forthin the following claims.

What I claim as new is:

1. A cathode ray tube system comprising a cathode to emit streams ofelectrons, a luminescent screen oppositely disposed from said cathode toreceive electrons therefrom, a cylindrical anode surrounding and alignedwith a portion of the path between said cathode and said screen, amulti-apertured disc supported within said anode one of the apertures ofwhich is axially aligned with said anode, the remaining apertures ofsaid disc surrounding said aligned aperture to minimize the collectionof electrons by said disc, electrode means between aid anode and saidscreen in the path of electrons passing through said remainingapertures, and means to maintain said anode and said electrode means atdiflerent positive potentials with respect to said cathode.

7 limit the,flow of electrons along said axis, a plurality of aperturessurrounding said aligned aperture to allow electrons to flow beyond saidcollecting electrode and means between said. cy-

lindrical anode and said target in the path of electrons passsingthrough said plurality of apertures.

3. A cathode ray system comprising a source of cathode rays, aluminescent screen oppositely disposed from said source, a pair ofaxially aligned cylindrical anodes between said source and said screento direct electrons along the axis of said anodes toward said screen, amulti-apertured disc in the anode nearer said source one of theapertures in said disc being axially aligned with said anodes, theremaining apertures surrounding the axially aligned aperture to allowthe principal portion of electrons not flowing through said aperture topass said disc in the direction of said screen, means between saidnearer anode and said screen to intercept the electrons passsing throughsaid remaining apertures, and means to maintain said anodes at difierentpositive potentials with respect to said cathode.

4. A cathode ray tube system comprising a cathode to emit streams ofelectrons, a cylindrical anode to receive the electrons emitted by saidcathode and focus said electron streams into a beam of'relatively largediameter, means within said anode to intercept electrons to form acentral relatively small diameter beam of electrons and a plurality ofstreams of electrons surrounding said central beam, a luminescent screento receive said central beam and means between said anode and saidscreen to intercept said plurality of streams of electrons and means tomaintain said anode and said last-mentioned means at positive potentialswith respect to said cathode.

5. A cathode ray tube system comprising a cathode to emit electrons, apair or axially aligned cylindrical anodes to direct the electrons intoa relatively large diameter electron beam within said anodes, meanswithin the anode nearer said cathode to intercept electrons from anannular section of said beam, said means allowing electrons within andwithout said annular section to flow toward said other anode, anapertured disc within said other anode to intercept the electrons ofsaid beam without said annular section, a high potential sourceconnected between said other anode and said cathode, a high resistanceconnected across said potential source and an intermediate connectionbetween said high resistance and said nearer anode to maintain saidnearer anode at a substantially constant potential with respect to saidcathode and said other anode. I

6. A cathode ray tube system comprising a cathode to emit streams oielectronaa cylindrical anode adjacent said cathode to direct electronsinto a relatively large diameter beam, a target oppositely disposed fromsaid cathode and anode, a second anode between said firstmentioned anodeand said target, means to maintain said anodes at difierent positivepotentials with respect to said cathode, means within said first anodeto cut an annular section from said large diameter beam but to passelectrons within and without said annular section in the direction ofsaid second anode whereby the electron current collected' by said firstanode is limited to the electrons comprising said annular section andthe potentialof said first anode is rendered-substantially constant, andmeans between said first anode and said target to collect the electronsof said beam without said annular section cut from said beam.

7. A cathode ray tube system comprising a cathode to liberate electrons,a target oppositely disposed from said cathode, a cylindrical anode todirect said electrons along diverging paths toward said target, a secondanode to direct a portion of said electrons flowing along said divergentpaths along converging paths toward said target, a single potentialmeans connected between said second anode and said cathode, a bleederresistance connected across said single potential source, an electricalconnection between said-cylindrical anode and an intermediate point onsaid bleeder resistance whereby the electron current collected by saidcylindrical anode flows through a portion of said bleeder, amulti-apertured disc within and connected to said cylindrical anode withone aperture axially aligned with said anodes and the remainingapertures surrounding said aligned aperture to limit the currentcollected by said disc whereby the current iiowing through the saidportion of said bieeder is minimized.

8. A. cathode ray tube system comprising a cathode to liberateelectrons, a cylindrical anode to accelerate the electrons from saidcathode, an apertured electrode to control the quantity of electronsflowing therefrom in the direction of said anode along converging pathsto a crossover point and thence along diverging paths within said anode,means to apply varying control potentials with respect to said cathodeto said apertured electrode inherently causing said.

cross-over point to move in a direction along the axis of saidcylindrical anode, a second anode to receive electrons from saidcylindrical anode, means including a potentiometer to maintain saidanodes positive with respect to the cathodeand said second anodepositive with respect to said cylindrical anode, a multi-apertured dischaving one aperture axially aligned with said anodes focused at apredetermined distance from said second anode irrrespective of'themovement of said cross-over point along said axis and means beyond saidcylindrical anode in a direction from said cathode in the path ofelectrons flowing through said remaining apertures to prevent saidlast-mentioned electrons reaching the point 01 focus at saidpredetermined distance.

J OHANNES DE GIER.

